Circuit for converting AC voltage into DC voltage

ABSTRACT

The invention relates to a circuit for converting AC voltage into DC voltage for a consumer (RL), in which the AC voltage supplied is passed through a large induction coil (L 50 ) and a rectifier (G). On the output side the rectifier (G) has two capacitors (C 1,  C 2 ) connected in parallel, which are separated by a diode (D). In addition, the capacitor C 1  positioned closer to the rectifier (G) is discharged at least in part via an active converter ( 20 ) in the second half of each power supply wave, as a result of which the current consumption is widened and the system harmonics are reduced. Furthermore, the induction coil (L 50 ) can be dimensioned correspondingly smaller, but can nevertheless meet the relevant standards.

[0001] The invention relates to a circuit for converting AC voltage into non-stabilized DC voltage for at least one consumer, the circuit comprising:

[0002] an AC voltage supply,

[0003] a rectifier module whose two inputs are connected to the AC voltage supply and to whose outputs are connected two output lines with external terminals for coupling at least one consumer,

[0004] at least one induction coil which is arranged between the AC voltage supply and the rectifier module.

[0005] A multiplicity of electrical or electronic devices require a DC voltage for their operation which voltage has to be provided or generated from the AC voltage supplied by the power supply system. Representative of such terminals are television sets (TV). The circuits for conversion of the AC voltage into DC voltage used in these devices must meet certain requirements concerning their conversion behavior, in particular in respect of the residual system harmonics content. Such requirements are not least the subject of standards, of which particularly EN 61000-3-2 can be named, which with effect from 2001 must be complied with in Europe by terminals that have a power input of more than 75 W.

[0006] In order to ensure that said quality standards are met, there are a number of different possibilities with varying optimum performance ranges. For high powers from approximately 500 W upwards active solutions predominate. In the lower power range, on the other hand, 50 Hz coils are often to be found. They are relatively cheap, but also relatively large and heavy. This size or this concentration of weight leads to problems with the available space on printed circuit boards or to problems with the impact resistance of the equipment manufactured.

[0007] Against this background the object of the present invention was to provide a circuit for conversion of AC voltage into DC voltage for at least one consumer, which guarantees sufficient quality of the conversion behavior with limited space and cost requirements.

[0008] This object is achieved by a circuit having the characteristics of claim 1. Advantageous embodiments are contained in the dependent claims.

[0009] The circuit for converting AC voltage into DC voltage for at least one consumer accordingly comprises the following components:

[0010] (a) An AC voltage supply, through which the AC voltage for conversion is provided.

[0011] (b) A prior-art rectifier module whose two inputs are connected to the AC voltage supply for the purpose of supplying an AC voltage and which has two DC voltage outputs from which corresponding output lines lead with external terminals to which at least one consumer can be connected.

[0012] (c) A main capacitor which is connected in parallel to a consumer to be connected between the two external connections of the rectifier module.

[0013] (d) At least one induction coil which is arranged in one of the connecting lines connecting the AC voltage supply to the rectifier module. The induction coil has the task of smoothing the recharging current or reducing the harmonic content of the recharging current.

[0014] The circuit is characterized in that it also comprises:

[0015] (e) A further capacitor which is connected in parallel to the main capacitor between the two outputs of the rectifier module.

[0016] (f) At least one diode which is connected in one of the output lines of the rectifier module and separates the main capacitor from the further capacitor, the diode being forward-biased relative to the DC voltage output of the rectifier module.

[0017] (g) An active converter whose input is connected between the diode and the rectifier module (and thus to a plate of the further capacitor) and arranged so that it discharges the further capacitor at least in part in the second half of each half-wave supplied by the AC voltage supply.

[0018] The behavior of the input current is altered in an advantageous manner through the further capacitor, which can have a relatively small capacitance, and through the discharge process via the active converter. There is particularly a reduction in the harmonic part. This can in turn be used for designing the induction coil to be correspondingly smaller, because the required inductance L can be selected to be smaller. In this way it is possible to use inductor coils with a drastically reduced size which can be used when space is limited on printed circuit boards and to avoid problems with the impact resistance of the devices. Compliance with the relevant standards is also guaranteed.

[0019] The active converter may be an up-controller or up-converter. The energy transported by the converter may either be fed to the main capacitor to supply power to the switched-mode regulator ICs of a downstream DC-DC converter or directly to a load on the mains-isolated side (secondary side) of a downstream DC-DC converter.

[0020] In the following the invention will be explained by way of example using the Figures, in which:

[0021]FIG. 1 shows a state-of-the-art circuit for supplying power to two consumers;

[0022]FIG. 2 shows a state-of-the-art circuit for supplying power to one consumer;

[0023]FIG. 3 shows the waveform of the current consumption in a circuit shown in FIG. 1 or 2;

[0024]FIG. 4 shows a circuit in accordance with the invention; and

[0025]FIG. 5 shows the waveform of the current consumption with the circuit as shown in FIG. 4.

[0026] The generation of DC voltages to supply power to electronic terminals from an AC mains voltage must meet certain minimum criteria in relation to the conversion of the AC voltage into DC voltage. For consumers with a power consumption of more than 75 W these requirements are, by way of example, set out in the future standard EN 61000-3-2. Various methods for meeting this standard have sought to generate the most exact sinusoidal input current possible. Such a sine waveform is still not specified as mandatory by the standard.

[0027] A much-used method of meeting the quality specifications consists of a passive solution using a so-called 50 Hz coil, which smoothes the input current. For this purpose, however, a high inductivity is necessary to comply with, for example, EN 61000-3-2. The corresponding coil is thus very big and heavy, which causes problems for its accommodation on a printed circuit board and for the impact testing of the devices. With the present invention a circuit is provided that allows the use of smaller coils while at the same time the quality specifications for conversion are complied with.

[0028] In standard EN 61000-3-2 a distinction is made between classes A to D, while the particular classification of a device depends on various characteristics (such as how easily the device can be moved). For many terminals such as TV sets, classes A to D can usually be applied, the particular classification being dependent on whether the signal form of the current consumed from an AC voltage source deviates or does not deviate by more than 5% from a reference curve defined in the standard. The reference curve is shown in FIG. 3 as a broken-line rectangular curve. If the signal form of a current consumed is more than 5% outside the reference curve, this device comes under class A. This class contains generous limiting values for the higher harmonics which are still allowed in the signal.

[0029]FIG. 1 shows a state-of-the-art circuit for supplying DC voltage to two consumers Rm, Ra, which is obtained from an AC voltage source by conversion. This circuit corresponds to that of FIG. 2 in which the two consumers Rm, Ra are connected in parallel to the outputs of the rectifier module G.

[0030]FIG. 2 shows in more detail the prior-art circuit for supplying DC voltage to a consumer RL. The power is taken from an AC voltage source AC and is converted by a rectifier module G into a non-stabilized or fluctuating (subject to ripple) DC voltage. The AC voltage is fed to the rectifier through the input terminals. In one of the input lines or in both input lines there is moreover a 50 Hz coil L50 for smoothing purposes. At the output terminals “+”, “−” of the rectifier module G the DC voltage can be tapped by the consumer RL. The two output lines to these terminals can be coupled via a smoothing capacitor C. Furthermore, an output line often contains a resistor R to achieve a limitation of the make current.

[0031] The curve resulting from a circuit in accordance with FIG. 1 or in accordance with FIG. 2 is shown in FIG. 3, in which the circuit is based on the following parameters: R=1.5 ohms, L50=32 mH, C=220 μF, Uin=230 V, f=50 Hz, Po=145 W. The broken line which is also shown in FIG. 3 corresponds to the reference curve of standard EN 61000-3-2, with which a classification from A and D is made. Since the signal form shown for the circuits in accordance with FIGS. 1 and 2 remains within the reference curve form, it must meet the requirements of class D in accordance with EN 61000-3-2.

[0032]FIG. 4 shows a circuit in accordance with the invention for supplying power to a consumer RL. In this circuit the required energy is taken from the mains supply W via the inputs (“˜”) of a rectifier module G. In this case it is taken via the 50 Hz coil L50 and a resistor R, which limits the input current at the switch-on time.

[0033] A small input capacitor C1 is initially provided on the output side of the rectifier module G between the DC voltage outputs (“+”, “−”). Furthermore, the outputs are connected via output lines to external terminals AA to which the consumer RL can be connected. A main capacitor C2 (electrolytic capacitor) is connected in parallel with the consumer RL to the external terminals AA. The main capacitor C2 is isolated from the input capacitor C1 by diode D forward-biased in relation to the rectifier.

[0034] Between the “+” output of the rectifier module G and the anode of diode D the input of an active converter 20 is coupled. Through the active converter 20 the input capacitor C1 is discharged every second half of a mains half-wave by or to a certain voltage. This leads to a change in the input current since this capacitor is now discharged earlier than the main capacitor. The discharge can take place by means of prior-art circuits such as boost or flyback (switching-regulator) circuits. The energy transported can either be added to the main capacitor or brought to the outputs X.

[0035]FIG. 5 shows the resulting signal pattern of the current consumption (50 Hz input current, one half-wave) when the circuit as shown in FIG. 4 is used. The input current has at its left base point in the Figure a spread that leads to a damping of the harmonics of the input current.

[0036] In the circuit shown in FIG. 4, the 50 Hz coil L50 can be reduced by more than 40%, if (L×I²/2) is used as an approximation for the size of the coil.

[0037] With all the proposals non-linear magnets can be used for further reduction of the size of the magnetic components. Key: AA External terminal AC AC voltage source C Smoothing capacitor C1 Capacitor C2 Main capacitor D Diode EA External terminal G Rectifier I Current IA Internal terminal L50 50 Hz coil R Ohmic resistance RL Consumer Rm, Ra Consumers in the first/second input path t Time W AC voltage supply 

1. A circuit for converting AC voltage into DC voltage for at least one consumer, the circuit comprising: an AC voltage supply (W), a rectifier module (G) whose two inputs are connected to the AC voltage supply and to whose two outputs (+, −) are connected two output lines with external terminals (AA) for coupling at least one consumer (RL), a main capacitor (C2) which is connected in parallel to a consumer to be connected between the two external terminals (AA) of the rectifier module (G), at least one induction coil (L50) which is arranged between the AC voltage supply (W) and the rectifier module (G), characterized in that the circuit also comprises: a further capacitor (C1) which is connected in parallel to the main capacitor (C2) between the two outputs (+, −) of the rectifier module (G), a diode (D) which is connected in one of the output lines of the rectifier module (G) and separates the main capacitor (C2) from the further capacitor (C1), an active converter (20) whose input is connected between the diode (D) and one of the outputs (+) of the rectifier module (G) and arranged so that it discharges the further capacitor (C1) at least in part in the second half of each half-wave supplied by the AC voltage supply (W).
 2. A circuit as claimed in claim 1, characterized in that the active converter (20) is an up-controller or up-converter or a flyback converter.
 3. A circuit as claimed in claim 1 or 2, characterized in that the output of the converter (20) is connected to the main capacitor (C2) so that it can convey the load taken from the capacitor (C1) to the main capacitor (C2).
 4. A circuit as claimed in at least one of claim 1 or 2, characterized in that the energy carried by the converter can be used to supply power to the ICs of the switched-mode regulator of a downstream DC-DC converter.
 5. A circuit as claimed in at least one of claim 1 or 2, characterized in that the energy carried by the converter can be applied directly to a load on the mains-isolated side of a downstream DC-DC converter. 